I think for fuel economy it would be about a wash, and would depend on how well your carburetor is tuned. The AA-1 carb on a 302 (might be on a 270 also) or a Carter would generally outperform a Rochester B on a 261, although you could swap a Carter to a 261. You might also want to run vacuum advance, some of the big GMC's don't have one. 261's do.
